Seismic expansion joint covers for buildings in geographic regions that are prone to earthquakes are of special designs that allow for movements of the building elements on either side of the expansion gap that are very much greater than the movements that occur as a result of thermal expansion and contraction. In that regard, buildings currently being built in earthquake-prone regions are usually supported on isolators that attenuate the intensities of shocks imparted to the building structure but increase the durations and magnitudes of the swaying motions of the structure as it deforms when forces due to the earthquake are imposed on its foundation supports. When a building is composed of two or more adjacent independent structural units, each structural unit is subject to movements in an earthquake that are different in direction, frequency and magnitude. This is the case, indeed, regardless of whether the units are mounted on isolators or not. Adjacent structural units of a building are, accordingly, subject to large relative movements having components toward and away from each other (perpendicular to the gap) and components parallel to the gap. Because the connections between structural units at expansion joints (which might better be termed "motion-absorbing gaps") occur at the perimeters of the structural units, the movements include small but meaningful relative displacements vertically and angularly between floor portions on opposite sides of gaps due to the rocking of the floors at the perimeter of the structural unit about a fulcrum in the region of the bottom center of the structural unit
Most seismic expansion joint covers follow traditional design philosophies that have long been applied to expansion joint covers that are not intended to sustain earthquakes; they use metal covers and various fastening systems to join the covers to frame members that are attached to the building members on either side of the expansion gap in such a way as to retain the covers in place in the gap during seismic events while permitting the large motions of the members. In a common fastening system, the cover is attached by bolts to the centers of spaced-apart bridge bars that span the gap with their ends sliding in trackways in the frame members. As the gap expands and contracts, the bridge bars pivot about the connecting bolts. An example of a seismic expansion joint cover system that uses bridge bars is found in U.S. Pat. No. 5,078,529 (Moulton, Jan. 7, 1992).
Previously known seismic expansion joint covers can sustain relatively weak seismic events but are severely damaged, often beyond repair, in severe seismic events, such as the one that occurred in January, 1994, just north of Los Angeles. An inspection of several installations in buildings near the 1994 earthquake of seismic expansion joint covers of various designs revealed bent and mangled covers, failed connectors, frames ripped from their anchors in the walls and floors, and damage to the walls adjacent the covers caused by impacts of the partially detached covers against the walls. Few of the inspected installations were repairable.